Spark-type casing perforator

ABSTRACT

An electrical spark jet for perforating well casings and formations wherein a shaped spark discharge is used to form a jet of fluid. The electrodes for the shaped spark discharge comprise a center electrode and an outer cylindrical electrode with an insulator having a cone-shaped indentation being disposed between them. The insulator is coated with a thin conductive film and a cone-shaped metal liner is mounted along but spaced from the conductive film. The liner is coated with an insulation to insure that the spark discharge occurs between the two electrodes and not the metal liner and the conductive film. The electrical spark jet can also be used to detonate a thin cone of high explosive behind the conical metal liner in order to form a stronger jet without the total explosive effect that would occur if a sufficiently large weight of explosive was used to insure initiation and propagation of the detonation.

United States Patent [72] Inventor Noyes D. Smith, Jr.

Bellaire, Tex.

21 1 App]. No. 864,726

[22] Filed Oct. 8, I969 [45] Patented Nov. 23, 1971 [73] Assignee Shell Oil Company New York, N.Y.

[54] SPARK-TYPE CASING PERFORATOR 8 Claims, 4 Drawing Figs.

Primary Examiner-David H. Brown Atl0rneys.l. H. McCarthy and Theodore E. Bieber ABSTRACT: An electrical spark jet for perforating well casings and formations wherein a shaped spark discharge is used to form a jet of fluid. The electrodes for the shaped spark discharge comprise a center electrode and an outer cylindrical electrode with an insulator having a cone-shaped indentation being disposed between them. The insulator is coated with a thin conductive film and a cone-shaped metal liner is mounted along but spaced from the conductive film. The liner is coated with an insulation to insure that the spark discharge occurs between the two electrodes and not the metal liner and the conductive film.

The electrical spark jet can also be used to detonate a thin cone of high explosive behind the conical metal liner in order to form a stronger jet without the total explosive effect that would occur ifa sufficiently large weight of explosive was used to insure initiation and propagation of the detonation.

BACKGROUND OF THE INVENTION The present invention relates to perforating devices and particularly perforating devices that are used to perforate casings and tubular members used to case boreholes, as for example, boreholes that are drilled in an attempt to recover hydrocarbon deposits. A conventional oil and gas well is completed by first drilling the borehole and then casing the borehole, after which the casing is perforated adjacent the producing formation to permit the hydrocarbon deposits to flow into the well.

In the past, it has been customary to use chemical explosives to drive various types of projectiles through the casing to perforate it. Alternatively, shaped charges have been used to form and drive a fluid jet. The shaped charges of chemical explosives include a metal liner which has a general conical shape, positioned on the end of a chemical explosive that is shaped to conform to the conical shape of the metal liner.

While perfora'tors using chemical explosives are successful, they have several disadvantages. For example, due to the violence of the explosions, all charges are detonated in rapid sequence before the shock wave from the first charge reaches its neighbor. Thus it is impossible to selectively perforate the casing. It is obvious that if only one of the devices were ignited, the resulting explosion would immediately set off or damage the remaining devices since electric detonators or caps are very sensitive to shock.

In order to establish a steady state detonation velocity and to provide for propagation to the edge of the conical charge, a large base charge and thick walls of the conical charge are required. Thus for a conventional chemical explosive jet penetration, the resulting external expanding shock frequently damages the casing. This effect is particularly important when multiple completions are attempted and perforation is through a liner as well as casing.

BRIEF SUMMARY OF THE INVENTION The present invention solves the above problems by utilizing a shaped spark discharge device in place of the presently used chemical explosive or as a detonator for a more efficient chemical explosive charge. The spare discharge device can be selectively triggered and due to the concentration of its energy does not destroy the remaining perforating devices or the perforating tool. Thus, the perforators can be individually controlled and positioned as desired.

The shaped spark discharge electrodes comprise a central electrode surrounded by a cylindrical electrode with an insulator positioned between the electrodes. The end of the insulator electrode assembly has a generally conical shaped indentation with a conically shaped liner being positioned adjacent the indentation, but spaced therefrom. The inner surface of the liner( adjacent the indented insulator) is provided with an insulating film, while a thin conducting film or coating is deposited on the end of the insulator to connect the central electrode to the outer cylindrical electrode. A generally cylindrical insulating piece is positioned between the outer periphery of the metal liner and the end of the cylindrical electrode to enclose a thin airspace between the end of the assembly and the metal liner. When it is desired to drive a jet through the casing, a large electrical charge is applied to the electrodes, as for example, the charge stored on a capacitor to create a spark discharge between the electrodes. The spark discharge, as a result of the conical shape of the end of the assembly, generates a relatively large pressure wave that propels the metal liner into a high-velocity penetrating jet which perforates the casing at the desired position.

To provide a stronger jet with minimum blast effect a highexplosive compound is used instead of the inert insulating layer behind the conical metal liner.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more easily understood from the following description of a preferred embodiment while taken in conjunction with the attached drawings in which:

FIG. I is an elevation view of a perforating device constructed according to this invention positioned in a well;

FIG. 2 is a cross-sectional view drawn to an enlarged scale of a shaped spark discharge and metal liner used in the perforating device as shown in FIG. 1;

FIG. 3 is a cross-sectional view of a shaped spark, conical explosive and metal liner also used in the perforating device as shown in FIG. 1; and

FIG. 4 is a cross-sectional view drawn to an enlarged scale of a modified spark discharge that can be used with the device of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the enclosed drawings, there is shown in FIG. 1 a perforating apparatus constructed according to this invention suspended in a well. More particularly, the perforating apparatus 11 is suspended in a well having a casing 10 installed therein. A plurality of perforating devices 12 are disposed at the lower end of the apparatus and are directed so as to perforate the casing in a plurality of directions. The perforating devices are coupled to the instrument case of the apparatus by means of a cable 13. Cable 13 in addition to having sufficient mechanical strength to support the perforating devices, should also have a plurality of electrical circuits. Preferably a separate circuit is used for each of the perforating devices so that they may be separately connected to the control circuit 14. Control circuit 14 contains controls which may be actuated from the surface to selectively fire the individual perforating devices in the desired sequence. Positioned above the control circuit is a charging circuit 15 that contains suitable capacitors for supplying the potential required to initiate the spark discharge. The downhole perforating device is coupled to the surface by means of a cable 16. Cable 16 has sufficient mechanical strength to support the downhole perforating device, as well as the electrical circuits for powering the downhole tool, including the power required for charging the capacitors in the charging circuit.

Referring now to FIG. 2, there is shown a cross section of a perforating device constructed according to this invention and drawn to an enlarged scale. More particularly, there is shown a perforating device, including a central electrode 20 surrounded by a cylindrical electrode 21. An insulator 22 surrounds the central electrode and in addition supports the cylindrical electrode 21. It should be noted that the end of the insulator is provided with a conical surface 23 and that the outer cylindrical electrode extends beyond the edge of the conical surface. The exact included angle of the conical surface is not important and may range from between 40 and The important feature is the use of the conical surface to generate a high-pressure jet when the spark is discharged as explained below. The conical surface of the insulator is provided with a thin conducting film 24 that connects the central electrode with the outer cylindrical electrode. A thin conducting film is required to initiate the discharge as explained below.

Positioned adjacent the end of the electrode assembly and spaced therefrom is a conical-shaped metal liner 30. The liner 30 is a conventional perforating liner and may be formed of any suitable metal preferably a ductile material. as for example, copper. The inner surface of the liner 30 is coated with an insulating film 31 that may be a conventional plastic material. The coated liner is spaced from the end of the electrode assembly by means of a cylindrical insulating sleeve 32 that bears against the end of the outer cylindrical electrode and the outer periphery of the liner. The insulating sleeve or ring 32 also serves to enclose the airspace 33 between the end of the electrode assembly and the metal liner.

The two electrodes are coupled by means of leads 40 and 41 to a storage capacitor 42. The storage capacitor 42 is, of course, of sufficient size to store the charge required for initiating the spark between the two electrodes. The spark is initiated by means of a spark switch 44 that is positioned between two terminals 43 which are in series circuit with the capacitor 42 and electrodes 20 and 21 via the conductor 41. The use of the spark switch to couple the capacitor to the electrodes insures a rapid discharge of the capacitor and thus a high-intensity spark discharge.

FIG. 3 shows a cross section of a modified spark jet in which the insulating film behind the metal liner is replaced with a cone-shaped explosive element 45 made of a high detonating velocity material with low shock sensitivity and good electrical insulating properties such as TNT, PETN or RDX instead of film 31. FIGS. 2 and 3 shown only the elements for generating high-speed jets by a cone-shaped electrical plasma or a chemical explosive detonated by the conical electrical plasma. It is to be understood that the mechanical mounting would provide protection from the borehole fluid and an airspace in front of the conical liner to permit the efficient formation of the jet, as is well known to the art for ordinary explosive jets.

Referring to FIG. 4, there is shown a modified form of the spark discharge device shown in FIG. 2. In the modified form, the central electrode of FIG. 2 is divided into a series of electrodes 50-52 and a single outer electrode 21. This effectively divides the conical surface into zones and provides a more uniform current distribution. The conical surface is coated with a conducting film 53 and the discharge occurs between each electrode 50-52 and the outer ring 21. Three capacitors 54-56 are provided for supplying the power to the electrodes. A power supply is provided for charging the capacitors while a spark switch 57 is used to discharge the capacitors.

When the capacitors are discharged, the current density increases at the areas of the zones increases. The number of rings can be increased to produce a more uniform current density. However, if the spacing between the rings becomes too small, the conducting layer between the rings must be uniform to insure that the discharge is uniform.

OPERATION The above perforating device is operated by securing a number of the devices to an assembly and then lowering it into a cased well as shown in FIG. 1. Of course, the device can also be used to perforate the formations in uncased wells. After the perforating devices are positioned adjacent either the section of casing or formation to be perforated, the devices can be selectively fired. The devices are fired by initiating a spark across the spark switch disposed in the circuit of each of the devices. Each device should have its own charging capacitor or a single charging capacitor can be used providing the control circuit includes suitable switches for connecting the charging capacitor to the individual perforating devices. As the spark is initiated, the capacitor will discharge between the central electrode and the outer cylindrical electrode due to the thin conducting coating along the conical surface of the insulator. This expanding electrical plasma will create a pressure wave. As a result of the conical surface the pressure wave will be focused in a direction along the axis of the central electrode forcing the liner along the axis into a high-speed jet, as with chemical explosives, either the casing or the formation,

or both.

In contrast to the shaped spark discharges utilized in this invention, conventional spark discharges radiate spherically in all directions and the majority of the energy is dissipated in directions other than the desired direction. The net result being that it would not form a high-speed jet of the liner to penetrate the casing.

From the above description, it can be appreciated that a perforating device has been provided in which the individual devices may be fired as desired or in any desired sequence.

The operation of the device with an explosive cone IS the same as described above for the conical electrically driven jet cone. Both with and without the explosive cone, the device is much safer than jet perforators which require sensitive explosive detonators to initiate the detonation of the relatively shock-insensitive high-explosive charge.

I claim as my invention:

1. A process for perforating a casing disposed in a wellbore comprising:

placing a generally cone-shaped metal liner adjacent the section of casing to be perforated; and

creating a spark discharge within an airgap adjacent said metal liner to form a fluid jet and drive it through said casing.

2. The process of claim 1 wherein said liner is positioned with its axis substantially perpendicular to the casing.

3. A perforating device for perforating well casings comprismg:

a cone-shaped metal liner, said liner being coated on its convex surface with an insulating film;

at least two electrodes, said electrodes being disposed to form a cone-shaped spark discharge, said liner being mounted adjacent said electrodes and spaced therefrom.

4. The perforating device of claim 3 wherein said pair of electrodes comprises:

a central electrode and a cylindrical outer electrode, said outer electrode surrounding said central electrode;

an insulator, said insulator being disposed between said electrodes.

5. The perforating device of claim 4 wherein said cylindrical electrode extends axially beyond said central electrode and said central electrode and said insulator is provided with a conically indented surface.

6. The perforating device of claim 5 wherein said insulator is provided with a thin conducting film that connects said central and cylindrical electrodes.

7. The perforating device of claim 6 wherein said liner and said insulator have substantially similarly shaped conical surfaces, said liner being axially spaced from said insulator to provide an airspace between said insulator and said liner and an insulating ring, said ring being positioned between the end of said cylindrical electrode and the outer periphery of the liner to enclose the airspace.

8. A perforating device for perforating well casings comprising: a cone-shaped metal liner,

at least two electrodes, said electrodes being disposed to form a cone-shaped spark discharge, said liner being mounted adjacent said electrodes and spaced therefrom, and explosive material is positioned between said insulator and said liner. 

1. A process for perforating a casing disposed in a wellbore comprising: placing a generally cone-shaped metal liner adjacent the section of casing to be perforated; and creating a spark discharge within an airgap adjacent said metal liner to form a fluid jet and drive it through said casing.
 2. The process of claim 1 wherein said liner is positioned with its axis substantially perpendicular to the casing.
 3. A perforating device for perforating well casings comprising: a cone-shaped metal liner, said liner being coated on its convex surface with an insulating film; at least two electrodes, said electrodes being disposed to form a cone-shaped spark discharge, said liner being mounted adjacent said electrodes and spaced therefrom.
 4. The perforating device of claim 3 wherein said pair of electrodes comprises: a central electrode and a cylindrical outer electrode, said outer electrode surrounding said central electrode; an insulator, said insulator being disposed between said electrodes.
 5. The perforating device of claim 4 wherein said cylindrical electrode extends axially beyond said central electrode and said central electrode and said insulator is provided with a conically indented surface.
 6. The perforating device of claim 5 wherein said insulator is provided with a thin conducting film that connects said central and cylindrical electrodes.
 7. The perforating device of claim 6 wherein said liner and said insulator have substantially similarly shaped conical surfaces, said liner being axially spaced from said insulator to provide an airspace between said insulator and said liner and an insulating ring, said ring being positioned between the end of said cylindrical electrode and the outer periphery of the liner to enclose the airspace.
 8. A perforating device for perforating well casings comprising: a cone-shaped metal liner, at least two electrodes, said electrodes being disposed to form a cone-shaped spark discharge, said liner being mouNted adjacent said electrodes and spaced therefrom, and explosive material is positioned between said insulator and said liner. 